a) Field of the Invention
The present invention relates to a lens system which has a field angle on the order of 20.degree. to 40.degree. and an F number on the order of 2.8, and is suited for use with the so-called electronic cameras and video cameras which use image pickup tubes and solid state image pickup devices.
b) Description of the Prior Art
Since the so-called electronic cameras, video cameras and the like which use image pickup tubes, image pickup devices and the like require disposing optical members such as low pass filters and infrared cut filters between lens systems and image pickup surfaces, lens systems to be used with these cameras must have back focal lengths which are long as compared with focal lengths thereof.
In case of an image pickup system which uses the so called color separating optical system for picking up three colors of RGB with a plurality of image pickup devices to improve qualities of colored images above all, it is necessary to interpose, in addition to the optical members mentioned above, optical elements such as mirrors and prisms for splitting an optical path, whereby a lens system must have a longer back focal length.
Therefore, a photographic lens system except for one which has a long focal length in particular, generally adopts the so-called retrofocus type composition which has a negative power and a positive power in order from the object side. Further, most of a silver salt cameras adopt telephoto type lens systems or those which have refractive power distributions symmetrical with regard to stops. This applies also to lens systems which have field angles on the order of 20.degree. to 40.degree..
Lens systems disclosed by Japanese Patents Kokai Publication No. Sho 61-200519, Kokai Publication No. Hei 2-118507, Kokai Publication No. Hei 5-134174 and Kokai Publication No. Hei 7-248447, for example, are known as conventional examples of lens systems which have the retrofocus type composition.
In the recent years where progresses have been made in manufacturing technologies, image pickup devices which are used in electronic cameras and video cameras have been shifting from image pickup tubes mainly to solid-state image pickup devices and there are now used solid-state image pickup devices on which pixels are arranged in numbers remarkably large as compared with sizes thereof.
Accordingly, cameras which use solid-state image pickup devices are now used for printing purposes though these cameras were not used conventionally for a reason that images obtained with these cameras were lower in qualities thereof than those of images obtained with silver salt cameras. However, it is difficult to manufacture a compact image pickup device which is to be used for these purposes even with the recent manufacturing technologies since it requires pixels in a number equal or larger than that of pixels specified by standards for highly precise televisions such as highvision which forms highly minute images. Under these circumstances, there have been developed image pickup devices which are enlarged and have larger number of pixels remaining unchanged in a size thereof as well as electronic cameras which uses these image pickup devices.
However, the enlarged image pickup devices have a defect that they can be manufactured in a smaller number from a wafer which has a definite area thereby requiring a higher manufacturing prime cost. Accordingly, there is a strong demand to accomplish both a compact configuration and a reduction of manufacturing costs at the same time by developing an image pickup device which is compact and has a large number of pixels.
However, a reduction in a size of pixels to be arranged on an image pickup device results in enhancement of the so-called Nyquist frequency, thereby producing a demand for extremely high optical performance of photographic lens systems.
On the other hand, the electronic cameras and video cameras which use electronic image pickup devices require lens systems having long back focal lengths, thereby obliging to select the retrofocus type composition which consists, in order from the object side, of a front negative lens group and a rear positive lens group, or has a power distribution asymmetrical with regard to a stop, thereby making it difficult to correct offaxial aberrations such as distortion and astigmatism. Further, since the front negative lens group diverges an axial light bundle, the rear positive lens group produces remarkable spherical aberration, thereby making it difficult to obtain a bright lens system.
Further, as the retrofocus type lens system has a longer back focal length, refractive powers of the negative front lens group and the positive rear lens group must be strengthened, thereby making it more difficult to correct the aberrations mentioned above.
Furthermore, color reproducibility and color moire are more problematic as images have higher qualities. Therefore, a multi-plate type camera represented by the so-called three-plate type camera which picks up the three primary colors RGB with the three image pickup devices is more desirable than the so-called single-plate type camera which uses an image pickup device having color filters arranged thereon in a mosaic or stripes.
For this reason, it is necessary to interpose the so-called color separating optical system between a photographic lens system and the image pickup devices, and the photographic lens system must have a longer back focal length, thereby making it more difficult to design the lens system.
Spherical aberration and chromatic aberration are not corrected sufficiently in the lens systems disclosed by Japanese Patents Kokai Publication No. Sho 61-200519, Kokai Publication No. Hei 2-118507 and Hei 5-134174. The lens system disclosed by Japanese Patent Kokai Publication No. Hei 7-248447 has remarkably high optical performance, but is extremely large and contrary to the purpose described above to configure image pickup systems compact while maintaining a compact size of image pickup devices.